Combustion engines such as diesel engines, gasoline engines, and gaseous fuel-powered engines are supplied with a mixture of air and fuel for combustion within the engine that generates a mechanical power output. In order to maximize the power output generated by this combustion process, the engine is often equipped with a divided exhaust manifold in fluid communication with a turbocharged air induction system.
The divided exhaust manifold increases engine power by helping to preserve exhaust pulse energy generated by the engine's combustion chambers. Preserving the exhaust pulse energy improves turbocharger operation, which results in a more efficient use of fuel. In addition, the turbocharged air induction system increases engine power by forcing more air into the combustion chambers than would otherwise be possible. This increased amount of air allows for enhanced fueling that further increases the power output generated by the engine.
In addition to the goal of maximizing engine power output and efficiency, it is desirable to simultaneously minimize exhaust emissions. That is, combustion engines exhaust a complex mixture of air pollutants as byproducts of the combustion process. And, due to increased attention on the environment, exhaust emission standards have become more stringent. The amount of pollutants emitted to the atmosphere from an engine can be regulated depending on the type of engine, size of engine, and/or class of engine.
One method that has been implemented by engine manufacturers to comply with the regulation of these exhaust emissions includes utilizing an exhaust gas recirculating (EGR) system. EGR systems operate by recirculating a portion of the exhaust produced by the engine back to the intake of the engine to mix with fresh combustion air. The resulting mixture has a lower combustion temperature and, subsequently, produces a reduced amount of regulated pollutants.
EGR systems require a certain level of backpressure in the exhaust system to push a desired amount of exhaust back to the intake of the engine. And, the backpressure needed for adequate operation of the EGR system varies with engine load. Although effective, utilizing exhaust backpressure to drive EGR can adversely affect turbocharger operation, thereby reducing the air compressing capability of the air induction system. The reduced air compressing capability may, in turn, reduce the engine's fuel economy and possibly the amount of power generated by the engine. Thus, a system is required that provides sufficient exhaust backpressure to drive EGR flow without adversely affecting turbocharger or engine operation.
U.S. Pat. No. 6,321,537 to Coleman et al. (“the '537 patent”) discloses a combustion engine utilizing an EGR system and a divided exhaust manifold together with a turbocharged air induction system. Specifically, the '537 patent describes an internal combustion engine having a plurality of combustion cylinders and an intake manifold in common fluid communication with the combustion cylinders. A first exhaust manifold and a second exhaust manifold are separately coupled with the combustion cylinders. A first variable geometry turbine is associated with the first exhaust manifold, and a second variable geometry turbine is associated with the second exhaust manifold. The EGR system includes a 3-way valve assembly disposed in fluid communication between the first exhaust manifold, the second exhaust manifold, and the intake manifold. The valve includes an inlet fluidly coupled with an inlet of the first variable geometry turbine, a first outlet fluidly coupled with an inlet of the second variable geometry turbine, and a second outlet fluidly coupled with the intake manifold.
During operation of the combustion engine described in the '537 patent, exhaust flows in parallel from the first exhaust manifold to the first variable geometry turbine and from the first exhaust manifold to the valve assembly. Spent exhaust from the first variable geometry turbine is mixed with exhaust from the second exhaust manifold and fed to the second variable geometry turbine. Spent exhaust from the second variable geometry turbine is discharged to the ambient environment. The valve assembly is selectively actuated to control a flow of exhaust from the two outlets. Exhaust flowing from the first outlet mixes with exhaust from the second exhaust manifold and flows into the second variable geometry turbine. Exhaust from the second outlet is cooled and then mixed with combustion air. The mixture of combustion air and exhaust is then transported to the inlet manifold. Controlling the amount of exhaust gas which is transported to the intake manifold provides effective exhaust gas recirculation within the combustion engine. Moreover, controlling the flow of exhaust to the second variable geometry turbine utilizes energy from the exhaust which is not transported to the intake manifold to drive the second variable geometry turbine.
Although the system in the '537 patent may adequately control exhaust gas recirculation in a turbocharged engine, it may be less than optimal. Specifically, the variable geometry nature of the two separate turbochargers may increase the cost, complexity, and controllability of the system.
The disclosed exhaust system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.